1. Field of the Invention
The present invention relates to an active material for non-aqueous electrolyte secondary battery, and to a non-aqueous electrolyte secondary battery comprising it.
2. Description of the Related Art
Recently, secondary batteries comprising a non-aqueous electrolyte solution have become widely utilized as high-power and high-energy density secondary batteries. The non-aqueous electrolyte secondary battery of the type is used as a main power source of mobile appliances and, in addition, it has become used also as a memory backup power source of mobile appliances. With the recent tendency toward increasing the energy density of the main power source of mobile appliances, the power source for memory backup is also desired to have an increased energy density.
As a secondary battery for memory backup, for example, a battery comprising lithium cobaltate (LiCoO2) as the positive electrode active material and spinel-structured lithium titanate (Li4Ti5O12) as the negative electrode active material has already been put into practical use. As another example, there is mentioned a battery structure in which lithium titanate is used as the positive electrode active material and a lithium-containing carbon material as the negative electrode.
However, the theoretical density and the theoretical weight-related capacity of lithium titanate that is used as the negative electrode active material or the positive electrode active material are 3.47 g/ml and 175 mAh/g, respectively; and this is problematic in that the energy density per volume is low.
Molybdenum dioxide reversibly reacts with lithium in the same potential region as that of lithium titanate, and its theoretical density and theoretical weight-related capacity are 6.44 g/ml and 210 mAh/g, respectively; and as compared with lithium titanate, this has a higher volume-related energy density. Accordingly, using molybdenum dioxide as a substitutive material for lithium titanate may increase the energy density per volume of batteries.
For example, in JP-A 2000-243454, proposed is a battery in which a lithium-containing cobalt oxide or a lithium-containing nickel oxide is used as the positive electrode active material, and molybdenum dioxide is used as the negative electrode active material.
A battery for backup is built in an appliance and mounted thereon as a battery, in which this is utilized with no protective circuit from the viewpoint of the mounting area and the cost. In general, a power from the main power source is supplied to thereto and in that condition, the battery works as fully charged; however, in case where the battery is kept under the condition with no power supply thereto from the main power source for a long time, the battery may be in an over-discharge condition. Accordingly, in any of the charged condition and the over-charged condition, the battery is desired to have excellent storage stability.
As so mentioned in the above, molybdenum dioxide is more excellent in the energy density per volume than lithium titanate. However, the present inventors' studies have revealed that a battery comprising molybdenum dioxide is problematic in that it could not secure sufficient storage stability.
For example, it has been known that a non-aqueous electrolyte secondary battery comprising lithium cobaltate as the positive electrode active material and molybdenum dioxide as the negative electrode active material shows rapid increase in the internal resistance during storage under over-discharge, and is therefore problematic in that it could not have sufficient over-discharge storage stability.